In an image forming apparatus such as an electrophotographic apparatus or electrostatic recording apparatus, an electrostatic latent image formed on a photosensitive member is first developed with a developer (toner). After the toner image formed on the photosensitive member is then transferred to a transfer medium such as paper as needed, the image is fixed thereto by any of various methods such as heating, pressing and use of solvent vapor.
As such image forming apparatus, a digital full-color copying machine and a digital full-color printer are put to practical use. In the digital full-color copying machine, after a colored image original is subjected to color separation through respective filters of blue, green and red, electrostatic latent images of the respective colors composed of a dot diameter of 20 to 70 μm corresponding to the colored image original are formed. These electrostatic latent images are developed with respective toners of yellow, magenta, cyan and black colors to form a full-color image making good use of a subtractive mixture effect.
In recent years, a demand for formation of full-color images with high image quality and high definition has more and more increased. In order to improve color reproducibility in particular, it is desirable that an image using color toners can be printed with a hue equivalent to printing with inks. As colorants for magenta toner, have heretofore been generally used organic pigments such as quinacridone pigments, perillene pigments, thioindigo pigments and β-oxynaphthenic anilide pigments (i.e., Naphthol AS pigments). Among these, the quinacridone pigments are widely used in that they are excellent in light resistance, heat resistance and transparency. With respect to the quinacridone pigments, it has been proposed to use a quinacridone pigment and another quinacridone pigment in combination or use a quinacridone pigment and another magenta pigment in combination for the purpose of improving toner properties.
For example, Japanese Patent Application Laid-Open No. 10-312088 has proposed a magenta toner making combined use of C.I. Pigment Red 122 and C.I. Pigment Red 57:1 as a colorant. Japanese Patent Application Laid-Open No. 2000-181144 has proposed a magenta toner making combined use of a dimethylquinacridone pigment (i.e., C.I. Pigment Red 122) and at least one red pigment having negatively charging ability or weakly positively charging ability as a colorant. Japanese Patent Application Laid-Open No. 2002-91086 has proposed a magenta toner making combined use of a quinacridone pigment and a Naphthol AS pigment or β-naphthol lake pigment as a colorant.
However, these magenta toners has involved a problem that magenta toner particles are easy to be broken by contact between the magenta toner particles or stress between a feed roll and a development roll, or between a development and a photosensitive member in an image forming apparatus. When the magenta toner particles are broken, the flowability and printing density of the magenta toner are lowered.
In addition, C.I. Pigment Red 57:1 that is a magenta pigment used in respective Examples in Japanese Patent Application Laid-Open No. 10-312088 and Japanese Patent Application Laid-Open No. 2002-91086 is poor in light resistance, and so the resulting image may be faded with time in some cases.
Further, since C.I. Pigment Red 5 and C.I. Pigment Red 209 that are magenta pigments used in respective Examples in Japanese Patent Application Laid-Open Nos. 2000-181144 and 2002-91086 are chlorine atom-containing compounds, there is a possibility that dioxin may be generated when paper, on which images have been formed, is incinerated.
Japanese Patent Application Laid-Open No. 2000-156795 discloses magenta toners containing C.I. Pigment Red 122, C.I. Pigment Violet 19 and C.I. Pigment Red 150 as colorants. However, these magenta toners are low in printing density and may cause fog in some cases.
As described above, when a quinacridone pigment is used as a magenta colorant, light resistance is improved compared with the case where another pigment is used. However, such a pigment tends to lower the printing density of the resulting magenta toner. In order to raise the printing density using the quinacridone pigment, it is necessary to use the pigment in a great amount. When the quinacridone pigment is used in a great amount, however, not only the cost of the resulting magenta toner is increased, but also the fixing ability of the toner is liable to lower.
On the other hand, it is investigated to use a Naphthol AS pigment alone without using any quinacridone pigment to provide a magenta toner. Naphthol AS pigments are classified into a mono-azo system and a dis-azo system. However, mono-azo pigments are generally widely adopted.
Japanese Patent Application Laid-Open Nos. 2000-81734 and 2002-182433 disclose magenta toner particles comprising C.I. Pigment Red 31 as a mono-azo pigment in Examples thereof. These magenta toner particles are applied to formation of images by a two-component development system as a two-component developer combined with carrier particles. However, the investigation by the present inventors has revealed that when these magenta toner particles are applied to formation of images by a non-magnetic one-component development system, not only the density of an image printed becomes low, but also hot offset occurs, and moreover storage stability becomes poor.
Japanese Patent Application Laid-Open No. 2002-72569 discloses a magenta toner comprising C.I. Pigment Red 150 as a mono-azo pigment. However, the investigation by the present inventors has revealed that not only this magenta toner can form only images having a hue far from a hue by printing with inks and is poor in color reproducibility, but also it is low in printing density and poor in low-temperature fixing ability and causes fog under high-temperature and high-humidity conditions.